FINDING QUASAR SUPERSTRUCTURES

We describe a graph theoretical method, using the minimal spanning tre e (MST), for finding candidates for quasar superstructures in quasar s urveys. The morphology of the candidate can be characterized by the no rmalized mean, m, and normalized standard deviation, sigma, of the edg e-lengths of the 2D and 3D MSTs associated with it. This allows compar ison with well-known distributions and the determination of the signif icance of the candidate. We apply this technique to six surveys, three of them with claims of detections of quasar superstructures. These th ree structures are recovered and in two of the other three surveys we find a new candidate for a superstructure, consisting of ten quasars, at z similar to 1.9, and a grouping of seven Seyfert galaxies at z sim ilar to 0.19. The former grouping has dimensions of similar to 120 x 9 0 x 20h(-3) Mpc(3) and the latter has dimensions of similar to 60 x 30 x 10h(-3) Mpc(3).